Design of integrated circuits involves integration of various circuit components on a semiconductor die. Integrated circuit components may include logical and memory elements, electrical components such as resistors, capacitors, inductors, etc. Frequently the need arises for functionality such as variable resistance, fuses, short-circuits, etc, in the design of these integrated, circuits. Conventionally, such need is answered by separate components for each of these functionalities. However, with shrinking device sizes and need for low cost integration solutions, design and integration of these various different components is expensive and inefficient due to limited real estate on semiconductor dies. This problem is exacerbated by associated design and test costs.
Magnetic storage elements such as MTJs are emerging in the industry as a smaller and faster alternative to conventional memory elements using electric charge. Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter).
FIG. 1 illustrates a conventional STT-MRAM bit cell 100. STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ) storage element 105, transistor 101, bit line 102 and word line 103. MTJ 105 is formed, for example, from at least two ferromagnetic layers (a pinned layer and a free layer), each of which can hold a magnetic field or polarization, separated by a thin non-magnetic insulating layer (tunneling harrier). Electrons from the two ferromagnetic layers can penetrate through the tunneling barrier due to a tunneling effect under a bias voltage applied to the ferromagnetic layers. The magnetic polarization of the free layer can be reversed so that the polarity of the pinned layer and the free layer are either substantially aligned or opposite. The resistance of the electrical path through MTJ storage element 105 will vary depending on the alignment of the polarizations of the pinned and free layers. This variance in resistance can be used to program and read STT-MRAM bit cell 100. STT-MRAM bit cell 100 also includes source line 104, sense amplifier 108, read/write circuitry: bipolar write pulse/read bias generator 106 and bit line reference 107. Those skilled in the art will appreciate the operation and construction of STT-MRAM bit cell 100 is provided merely as an example.
As can be seen, MTJ storage element 105 can exhibit at least the two resistance values based on respective alignment of the pinned layer and the free layer. However, in conventional integrated circuits, MTJ elements such as MTJ storage element 105 fail to find application outside their role as storage elements. More specifically, existing solutions do not extend MTJ elements for use in other circuit components such as variable resistance devices, short-circuit elements, fuses, etc. As a result, existing solutions involve expensive reuse and redundancy to realize these various circuit components.
Accordingly, there is a need in the art for multiple-time programmable devices which can be programmed and configured for reuse in order to realize the various circuit functionalities in a single device.